I wanted to have a option for a system rescue environment on hand without residing to an additional USB stick. I also wanted to make use of the GRUB capability of booting ISO Images and went with https://www.system-rescue.org .
Since grml-rescueboot is not available on Fedora, I ... * downloaded 42_grml from the projects git * placed it on /etc/grub.d/ * gave it 744 permissions * put the downloaded ISO on /boot/grml/ * optionally: * since I wanted to have a look at iommu, I created the file /etc/default/grml-rescueboot and added CUSTOM_BOOTOPTIONS='iommu=1 iommu_amd=on' as described here
Creating the new GRUB worked, but booting failed with an error regarding missing modules.
The lesson learned, GRUB will use the modules from the currently used architecture, in our case i386-xen_pvh, for every menuentry.
Since we will be booting outside of xen, we need x86/64 and because GRUB wont use these modules, we need to place them on the boot folder.
qubes-dom0-update grub2-efi-x64-modules
mkdir -p /boot/efi/EFI/qubes/x86_64-efi/
cp -r /usr/lib/grub/x86_64-efi/* /boot/efi/EFI/qubes/x86_64-efi/
grub2-mkconfig -o /boot/efi/EFI/qubes/grub.cfg